It has been suggested to add BF3 or a complex containing BF3 (a BF3 complex) to electrolyte solutions of non-aqueous lithium secondary batteries. For example, Patent Document 1 teaches that capacity reduction rates can be lowered by addition of BF3, HBF and their complexes as capacity reduction rate-suppressing additives, that capacity reduction rates were lowered by addition of BF3-diethyl carbonate complex, and that addition of BF3-diethyl ether complex may contribute to lowering of capacity reduction rates under certain test conditions. Patent Document 2 teaches adding a Werner complex of BF3 to an electrolyte solution to increase cycle properties (capacity retention rates), with BF3-diethyl carbonate complex being used in worked examples. Patent Documents 3 and 4 are technical literature related to techniques for obtaining a highly anti-oxidative electrolyte solution by use of a BF3-ether complex as a solvent for the electrolyte solution.